1. Field of the Invention
The present invention relates to an optical disc apparatus, and in particular to an optical disc apparatus for finding an accurate tracking error signal for an optical disc.
2. Description of the Related Art
An optical disc is known as an information recording medium for storing a large amount of data. An optical disc can store information on tracks thereof, and also allow information recorded thereon to be reproduced. An optical disc apparatus is capable of mounting an optical is other on and is used for recording information on the optical disc and/or reproducing information stored on the optical disc. In order to allow the optical disc apparatus to record information to or reproduce information from an appropriate track accurately, a laser beam needs to accurately follow the tracks on the optical disc. The operation of the laser beam to follow the tracks on the optical disc is referred to as xe2x80x9ctrackingxe2x80x9d. A tracking error signal shows whether the laser beam is accurately following the tracks on the optical disc.
Hereinafter, a conventional optical disc apparatus and a tracking error signal provided by the conventional optical disc apparatus will be described.
FIG. 10A shows a conventional optical disc apparatus 1000. Laser light emitted by a laser light source 1010 is converged on an optical disc 1070 through an optical system 1015. The light reflected by the optical disc 1070 is detected by a photodetector 1050. Based on a result detected by the photodetector 1050, a control device 1085 controls an element or elements among the light source 1010, the optical system 1015, and the optical disc 1070 as necessary. The optical system 1015 includes, for example, a polarizing beam splitter 1020 having a splitting face 1025, a collimator lens 1030, a quarter-wave plate 1042, a reflecting mirror 1040, and an objective lens 1060.
A more specific operation of the optical disc apparatus 1000 will be described.
Laser light emitted by the light source 1010 is incident on the polarizing beam splitter 1020, transmitted through the splitting face 1025 of the polarizing beam splitter 1020, and then converted into parallel light by the collimator lens 1030. The parallel light, which is linearly polarized (P wave) is converted into circular polarization, by the quarter-wave plate 1042, and then reflected by the reflecting mirror 1040. The reflected light is converged by the objective lens 1060 on a signal face 1074 of the optical disc 1070.
The optical disc 1070 has the signal face 1074 between a substrate 1072 and a protection film 1076. The signal face 1074 has pits (or grooves) formed in a diameter direction of the optical disc 1070 (indicated by arrow X). The pits each have a depth d and a width w, and are arranged at a pitch p. The diameter direction of the optical disc 1070 is perpendicular to the direction of the light incident on the optical disc 1070 and parallel to the sheet of paper of FIG. 10A.
The light reflected by the signal face 1074, which is circular polarization, is transmitted through the objective lens 1060, reflected by the reflecting mirror 1040, and then converted into linear polarization (S wave) by the quarter-wave plate 1042. The light is made convergent by the collimator lens 1030, reflected by the splitting face 1025 of the polarizing beam splitter 1020, and then collected on the photodetector 1050 as light 1080. Based on a signal detected by the photodetector 1050, the control device 1085 controls an element or elements among the light source 1010, the optical system 1015, and the optical disc 1070 as necessary.
In FIG. 10A, reference numeral 1210 represents an optical axis of the optical disc apparatus 1000.
FIG. 10B shows a structure of the photodetector 1050. The photodetector 1050 includes sub-photodetectors 1050A and 1050B. A separation line 1051 shows the border between the sub-photodetectors 1050A and 1050B. The sub-photodetector 1050A and 1050B each provide a respective light amount. A tracking error signal 1091s (TE1 signal) is obtained by subjecting the light amounts provided by the sub-photodetectors 1050A and 1050B to subtraction performed by a subtracter 1091. A reproduction signal 1092 is obtained by subjecting the light amounts provided by the sub-photodetectors 1050A and 1050B to addition performed by an adder 1092. The separation line 1051 substantially equally divides a convergence spot 1081 on the photodetector 1050. The control device 1085 controls an element or elements among the light source 1010, the optical system 1015, and the optical disc 1070 as necessary, so as to make the level of the TE1 signal zero in order to eliminate a tracking error.
FIG. 11A shows another conventional optical disc apparatus 1100. Laser light emitted by a laser light source 1110 is converged on an optical disc 1170 through an optical system 1115. The light reflected by the optical disc 1170 is detected by a photodetector 1190, Based on a result detected by the photodetector 1190, a control device 1185 controls an element or elements among the light source 1110, the optical system 1115, and the optical disc 1170 as necessary. The optical system 1115 includes, for example, a collimator lens 1130, a quarter-wave plate 1142, a polarizing holographic element 1145, and an objective lens 1160.
A more specific operation of the optical disc apparatus 1100 will be described.
Laser light emitted by the light source 1110 is converted into parallel light by the collimator lens 1130 and incident on the polarizing holographic element 1145.
The polarizing holographic element 1145 is integrated into a lens holder 1165 together with the objective lens 1160. The polarizing holographic element 1145 has the quarter-wave plate 1142. A surface of the polarizing holographic element 1145 is a polarizing holographic face 1150.
The light, which is linear polarization (P wave) incident on the polarizing holographic element 1145 is transmitted through the polarizing holographic face 1150 and converted into circular polarization by the quarter-wave plate 1142, collected by the objective lens 1160, and then converged on a signal face 1174 of the optical disc 1170.
The optical disc 1170 has the signal lace 1174 between a substrate 1172 and a protection film 1176. The signal face 1174 has pits (or grooves) formed in a rotation direction of the optical disc 1170. The pits each have a depth d and a width w, and arranged at a pitch p.
The light reflected by the signal face 1174, which is circular polarization, is transmitted through the objective lens 1160, converted into linear polarizatlon (S wave) by the quarter-wave plate 1142, and then diffracted by the polarizing holographic face 1150. The diffraction light is transmitted through the collimator lens 1130 and incident on the photodetector 1190. Based on a signal detected by the photodetector 1190; the control device 1185 controls an element or elements among the light source 1110, the optical system 1115, and the optical disc 1170 as necessary.
FIG. 11B shows a structure of the polarizing holographic face 1150. The polarizing holographic face 1150 includes two areas 1150a and 1150b which are separated from each other by a separation line 1152. The light reflected by the optical disc 1170 is substantially equally divided into two by the separation line 1152.
FIG. 11C shows a structure of the photodetector 1190. The photodetector 1190 includes two sub-photodetectors 1190A and 1190B separated from each other by a separation line 1191. The light diffracted by the area 1150a (FIG. 11B) of the polarizing holographic face 1150 is collected on the sub-photodetector 1190A as a spot 1181a. The light diffracted by the area 1150b (FIG. 11B) of the polarizing holographic face 1150 is collected on the sub-photodetector 1190B as a spot 1181b. The sub-photodetectors 1190A and 1190B each provide a respective light amount. A tracking error signal 1101s (TE2 signal) is obtained by subjecting the light amounts provided by the sub-photodetectors 1190A and 1190B to subtraction performed by a subtracter 1101. A reproduction signal 1102B is obtained by subjecting the light amounts provided by the sub-photodetectors 1190A and 1190B to addition performed by an adder 1102. The control device 1185 controls an element or elements among the light source 1110, the optical system 1115, and the optical disc 1170 as necessary, so as to make the level of the TE2 signal zero in order to eliminate a tracking error.
The tracking error signals (TE1 signal and TE2 signal) obtained by the conventional optical disc apparatuses 1000 and 1100 have the following problems. First, the tracking error signal obtained by the conventional optical disc apparatus 1000 (TE1 signal) will be described.
Generally in the optical disc 1000, in which the control device 1085 performs tracking control, when the optical disc 1070 vibrates with respect to the center thereof, the objective lens 1060 follows the vibration and is shifted in the diameter direction K (FIG. 10A).
FIG. 12 (parts (a) through (d)) shows light intensity distributions of a cross-section of the optical disc 1070 when a central axis 1220 (part (e)) of the objective lens 1060 is shifted rightward by distance X with respect the optical axis 1210 of the optical disc apparatus 1000 (FIG. 1). The cross-section is taken along the diameter direction of the optical disc apparatus 1070. Part (e) schematically shows the positional relationship between the optical axis 1210 and the central axis 1220 of the objective lens 1060.
In FIG. 12, part (a) shows a light intensity distribution 1231 before the light emitted by the light source 1030 is transmitted through the objective lens 1060. The light intensity distribution 1231 exhibits a Gaussian distribution with the optical axis 1210 as the center. At this point, as shown in part (e), the central axis 1220 of the objective lens 1060 is shifted by distance X with respect to the optical axis 1210 of the optical disc apparatus 1000.
Part (b) shows a light intensity distribution 1232 after the light is transmitted through the objective lens 1060. When the objective lens 1060 has a radius (aperture radius) of length r, the light intensity distribution 1232 is zero at a position farther than distance r from the central axis 1220 of the objective lens 1060. In other words, the light outer aperture rims 1240 and 1250 of the objective lens 1060 are shielded.
Part (c) shows a light intensity distribution 1233 after the light is reflected by the optical disc 1070 and before being incident on the objective lens 1060. A central axis 1215 of the light reflected by the optical disc 1070 is shifted rightward by distance X with respect to the central axis 1220 of the objective lens 1060. In other words, the central axis 1215 of the light reflected by the optical disc 1070 is shifted rightward by distance 2X with respect to the optical axis 1210 of the optical disc apparatus 1000. The light intensity distribution 1233 is spread in the diameter direction of the optical disc 1070 due to the diffraction at the pits on the signal face 1074 of the optical disc apparatus 1070.
Part (d) shows a light intensity distribution 1234 after the light is transmitted through the objective lens 1060. As in part (b), the light outside the aperture rime 1240 and 1250 of the objective lens 1060 is shielded.
When distance X is zero, the tracking of the optical disc 1070 is accurately controlled by controlling the level of the tracking error signal (TE1 signal) obtained by the photodetector 1050 (FIG. 10B) to be zero. However, when distance X is not zero, a tracking offset is generated.
As described above, the tracking error signal (TE1 signal) obtained by the photodetector 1050 (FIG. 10B) shows a difference in the light amounts detected by the sub-photodetectors 1050A and 1050B. When a distance X exists between the optical axis 1210 and the central axis 1220 of the objective lens 1060, the light amount detected by the sub-photodetector 1050A corresponds to an area of a pattern ABCD formed by connecting points A, B, C and D (part (d)), and the light amount detected by the sub-photodetector 1050B correspond to an area of a pattern CDEF formed by connecting points C, D, E and P.
The tracking error signal (TE2 signal) obtained by the photodetector 1190 of the optical disc apparatus 1100 (FIG. 11A) is also shifted in a similar manner when there is a distance between an optical axis of the optical disc apparatus 1100 and a central axis of the objective lens 1160 for the following reason.
The tracking error signal (TE2 signal) obtained by the photodetector 1190 (FIG. 11C) shows a difference in the light amounts detected by the sub-photodetectors 1190A and 1190B. When a distance X exists between the optical axis of the optical disc apparatus 1100 and the central axis of the objective lens 1160, the light amount detected by the sub-photodetector 1190A correspond to an area of a pattern formed by connecting points A, B, Cxe2x80x2 and Dxe2x80x2 (part (d)), and the light amount detected by the sub-photodetector 1090B correspond to an area of a pattern formed by connecting points Cxe2x80x2, Dxe2x80x2, E and F. The tracking error signal provided by the photodetector 1190 (TE2 signal) is not offset as much as the tracking error signal provided by the photodetector 1050 (TE1 signal) but is still offset significantly.
FIG. 13A is a graph illustrating the degree of asymmetry of the waveform of the tracking error signal when the laser light crosses the pits (when tracking is off). In FIG. 13A, distance X between the optical adds 1210 of the optical disc apparatus 1000 and the central axis 1220 of the objective lens 1060 is assumed to be 100 xcexcm. The degree of asymmetry is represented as contours. The degree of asymmetry is obtained by expression (Hxe2x88x92L)/(H+L), where H is a level of the signal output (indicated by reference numeral 1300) shown in FIG. 13B above the ground level GND, and L is a level of the signal output shown in FIG. 13B below the ground level GND.
In FIG. 13A, the horizontal axis represents the width of the pits w of the optical disc 1070, and the vertical axis represents the depth of the pits (dxc3x97refractive index of the substrate 1072 of the optical disc 1070, see FIG. 10A). The parameters for the calculation obtained for the results shown in FIG. 13A are as follows: the numerical aperture (NA) of the objective lens 1060=0.60; the wavelength xcex of the light source 1010=0.66 xcexcm; the pitch (P) of the pits of the optical disc 1070=0.74 xcexcm. At point R (where the width w of the pits is 0.30 xcexcm and the depth of the pits is xcex/10), the degree of asymmetry of the tracking error signal is 0.52. This corresponds to the difference between the areas of the pattern ABCD and the pattern CDEF shown in part (d) of FIG. 12. As can be appreciated, in the optical disc apparatus 1000 including the photodetector 1050, the central axis 1220 of the objective lens 1060 is shifted with respect to the optical axis 1210 of the optical disc apparatus 1000 in the direction of arrow X (FIG. 1A). As a result, a significant degree of asymmetry of the tracking error signal occurs, and therefore control of tracking becomes unstable. While tracking control is performed, very large off-track may be undesirably generated. This causes a tracking error signal from an adjacent track to be leaked (i.e., crosstalk is increased) and deteriorates the reproduction performance, or causes a part of a signal mark of an adjacent track to be overwritten or erased.
FIG. 14 is a graph illustrating the degree of asymmetry of the waveform of the tracking error signal generated when the photodetector 1190 in the optical disc apparatus 1000 issused. The conditions are the same as above. At point R (where the width w of the pits is 0.30 xcexcm and the depth of the pits is xcex/10), the degree of asymmetry of the tracking error signal is 0.18. This corresponds to the difference between the areas of the pattern ABCxe2x80x2Dxe2x80x2 (and the pattern Cxe2x80x2Dxe2x80x2EF shown in part (d) of FIG. 12. The degree of asymmetry is lower than that provided by the photodetector 1050 but is still sufficiently large to cause the unstable control of tracking, a significant control error (off-track), and other problems.
An optical disc apparatus capable of mounting an optical disc according to the present invention includes a light source for emitting light; an objective lens for collecting the light emitted by the light source on the optical disc; a first light distribution section integrally movable with the objective lens, the first light distribution section including a first area and a second area, the first light distribution section outputting the light reflected by the optical disc and transmitted through the first area or the second area as transmission light, outputting the light reflected by the optical disc and diffracted by the first area as first diffraction light, and outputting the light reflected by the optical disc and a diffracted by the second area as second diffraction light; a transmission light detection section for detecting the transmission light and outputting a TE1 signal indicating an offset of the detected transmission light; a first diffraction light detection section for detecting the first diffraction light and the second diffraction light, and outputting a TE2 signal indicating a difference between a light amount of the detected first diffraction light and a light amount of the detected second diffraction light; and a control device for generating a tracking error signal for the optical disc based on the TE1 signal and the TE2 signal.
In one embodiment of the invention, the optical disc apparatus further includes a second light distribution section for directing the transmission light toward the transmission light detection section, and directing the first diffraction light and the second diffraction light toward the first diffraction light detection section.
In one embodiment of the invention, the transmission light detection section includes a first sub-transmission light detection section and a second sub-transmission light detection section. First transmission light is defined as part of the transmission light, which is detected by the first sub-transmission light detection section, and second transmission light is defined as a part of the transmission light, which is detected by the second sub-transmission light detection section. The offset of the transmission light is defined as a difference between a light amount of the first transmission light and a light amount of the second transmission light.
In one embodiment of the invention, the first diffraction light detection section includes a first sub-diffraction light detection section for detecting the first diffraction light and a second sub-diffraction light detection section for detecting the second diffraction light.
In one embodiment of the invention, the control device obtains the tracking error signal by TE2xe2x88x92kxc3x97TE1.
In one embodiment of the invention, the transmission light detection section includes a third area and a fourth area. The first sub-transmission light detection section is provided in the third area, and the second sub-transmission light detection section is provided in the fourth area. A border between the third area and the fourth area is parallel to a rotation direction of the optical disc.
In one embodiment of the invention, the first diffraction light detection section includes a fifth area and a sixth area. The first sub-diffraction light detection section is provided in the fifth area, and the second sub-diffraction light detection section is provided in the sixth area. A border between the fifth area and the sixth area is parallel to a rotation direction of the optical disc.
In one embodiment of the invention, the control device updates a value of k in accordance with a logical product of a numerical aperture (NA) of the objective lens and a pitch (P) of the optical disc in a diameter direction of the optical disc (NAxc3x97P).
In one embodiment of the invention, a value of k is 0.5xc3x97S2/S1 or less, wherein S1 is a light amount of the transmission light detected by the transmission light detection section, and S2 is a light amount of the diffraction light detected by the first diffraction light detection section.
In one embodiment of the invention, the control device sets the value of k at zero when the logical product of the numerical aperture (NA) of the objective lens and the pit pitch (P) of the optical disc in the diameter direction of the optical disc (NAxc3x97P) is 0.9 times or more of the wavelength of the light incident on the optical disk.
In one embodiment of the invention, the control device sets a value of k so that an average output level of TE2xe2x88x92kxc3x97TE1 is substantially zero when the control device shifts the objective lens in a diameter direction of the optical disc without performing tracking control.
In one embodiment of the invention, the optical disc apparatus further includes an aberration section for providing the transmission light with an aberration. The tranismission light detection section includes a third area, a fourth area, a seventh area and an eighth area. The first sub-transmission light detection section is provided in the third area. The second sub-transmission light detection section is provided in the fourth area. The third sub-transmission light detection section is provided in the seventh area. The fourth sub-transmission light detection section is provided in the light area. A border between the third area and the fourth area is parallel to a rotation direction of the optical disc. A border between the third area and the eighth area is parallel to a diameter direction of the optical disc. A border between the fourth area and the seventh area is parallel to a diameter direction of the optical disc. A border between the seventh area and the eighth area is parallel to a rotation direction of the optical disc. The third area is orthogonal with respect to the seventh area. The fourth area is orthogonal with respect to the eighth area. The control device obtains a focusing error signal for the optical disc based on a difference between a sum of a light amount of the transmission light provided with the aberration and detected by the first sub-transmission light detection section and a light amount of the transmission light provided with the aberration and detected by the third sub-transmission light detection section, and a sum of a light amount of the transmission light provided with the aberration and detected by the second sub-transmission light detection section and a light amount of the transmission light provided with the aberration and detected by the fourth sub-transmission light detection section.
In one embodiment of the invention, the first light distribution section includes a ninth area and a tenth area. The first light distribution section outputs the light reflected by the optical disc and diffracted by the ninth area of the first light distribution section as third diffraction light, and outputs the light reflected by the optical disc and diffracted by the tenth area of the first light distribution section as fourth diffraction light. The first diffraction light detection section includes a first sub-diffraction light detection section, a second sub-diffraction light detection section, a third sub-diffraction light detection section: a fourth sub-diffraction light detection section, a fifth sub-diffraction light detection section, and a sixth sub-diffraction light detection section. The first diffraction light is detected by the first sub-diffraction detection section and the second sub-diffraction detection section. The second diffraction light is detected by the fifth sub-diffraction detection section and the sixth sub-diffraction detection section. The third diffraction light is detected by the fourth sub-diffraction detection section and the fifth sub-diffraction detection section. The fourth diffraction light is detected by the second sub-diffraction detection section and the third sub-diffraction detection section. The control device obtains a focusing error signal for the optical disc based on a difference between a total light amount of the diffraction light detected by the first sub-diffraction light detection section, the third sub-diffraction light detection section and the fifth sub-diffraction light detection section, and a total light amount of the diffraction light detected by the second sub-diffraction light detection section, the fourth sub-diffraction light detection section and the sixth sub-diffraction light detection section.
In one embodiment of the invention, the optical disc apparatus further includes a second diffraction light detection section. The first light distribution section outputs the light, reflected by the optical, disc and diffracted by the first area of the first light distribution section separately from the first diffraction light, as fifth diffraction light, and outputs the light, reflected by the optical disc and diffracted by the second area of the first light distribution section separately from the second diffraction light, as sixth diffraction light. The second diffraction light detection section includes a seventh sub-diffraction light detection section and an eighth sub-diffraction light detection section. The control device obtains a focusing error signal for the optical disc based on a difference between a light amount of the fifth diffraction light detected by the seventh sub-diffraction light detection section and alight amount of the sixth sub-diffraction light detected by the eighth sub-diffraction light detection section.
In one embodiment of the invention, the first light distribution section includes a holographic element having a pattern having sawtooth-lie or step-like shape including three or more steps, the pattern being continuous over sequential cycles. The first light distribution section outputs the light, reflected by the optical disc and diffracted by the first area of the first light distribution section separately from the first diffraction light, as fifth diffraction light, and outputs the light, reflected by the optical disc and diffracted by the second area of the first light distribution section separately from the second diffraction light, as sixth diffraction light. A light amount of the first diffraction light and a light amount of the fifth diffraction light both output by the first light distribution section are different from each other, and a light amount of the second diffraction light and a light amount of the sixth diffraction light both output by the first light distribution section are different from each other.
In one embodiment of the invention, the first diffraction light and the second diffraction light output by the first light distribution section are positive first order diffraction light, and the fifth diffraction light and the sixth diffraction light output by the first light distribution section are negative first order diffraction light.
In one embodiment of the invention, a light amount of the negative first order diffraction light is substantially zero.
In one embodiment of the invention, a light amount output by the first light distribution section is largest for the positive first order diffraction light, second largest for the transmission light, and smallest for the negative first order diffraction light.
In one embodiment of the invention, a light amount output by the first light distribution section is largest for the transmission light, second largest for the positive first order diffraction light, and smallest for the negative first order diffraction light.
In one embodiment of the invention, a light amount output by the first light distribution section is largest for the transmission light, second largest for the negative first order diffraction light, and smallest for the positive first order diffraction light.
In one embodiment of the invention, the optical disc apparatus further includes a second diffraction light detection section. The first light distribution section includes a ninth area and a tenth area. The first light distribution section outputs the light reflected by the optical disc and diffracted by the ninth area of the first light distribution section as third diffraction light, outputs the light reflected by the optical disc and diffracted by the tenth area of the first light distribution section as fourth diffraction light, outputs the light, reflected by the optical disc and diffracted by the first area of the first light distribution section separately from the first diffraction light, as fifth diffraction light, and outputs the light, reflected by the optical disc and diffracted by the second area of the first light distribution section separately from the second diffraction light, as sixth diffraction light. The second diffraction light detection section includes an eleventh area, a twelfth area, a thirteenth area, a fourteenth area, a fifteenth area, and a sixteenth area. A seventh sub-diffraction light detection section is provided in the eleventh area. An eighth sub-diffraction light detection section i s provided in the twelfth area. A ninth sub-diffraction light detection section is provided in the thirteenth area. A tenth sub-diffraction light detection section is provided in the fourteenth area. An eleventh subsidization light detection section is provided in the fifteenth Area. A twelfth sub-diffraction light detect Ion sect Ion is provided in the sixteenth area. The third diffraction light lo detected by the seventh sub-diffraction light detection section and the eighth sub-diffraction light detection section. The fourth diffraction light is detected by the is eleventh sub-diffraction light detection section and the twelfth sub-diffraction light detection section. The fifth diffraction light is detected by the tenth sub-diffraction light detection section and the eleventh sub-diffraction light detection section. The sixth diffraction light is detected by the eighth sub-diffraction light detection section and the ninth sub-diffraction light detection section. The control device obtains a focusing error signal for the optical disc based on a difference between a total light amount of the diffraction light detected by the seventh sub-diffraction light detection section, the ninth sub-diffraction light detection section and the eleventh sub-diffraction light detection section, and a total light amount of the sub-diffraction light detected by the eighth sub-diffraction light detection section, the tenth sub-diffraction light detection section and the twelfth sub-diffraction light detection section.
In one embodiment of the invention, the optical disc apparatus further includes a second diffraction light detection section. The first light distribution section includes a ninth area and a tenth area. The first light distribution section outputs the light reflected by the optical disc and diffracted by the ninth area of the first light distribution section as third diffraction light, outputs the light reflected by the optical disc and diffracted by the tenth area of the first light distribution section as fourth diffraction light, outputs the light, reflected by the optical disc and diffracted by the first area of the first light distribution section separately from the first diffraction light, as fifth diffraction light, and outputs the light, reflected by the optical disc and diffracted by the second area of the first light distribution section separately from the second diffraction light, as sixth diffraction light. The second diffraction light detection section includes an eleventh area, a twelfth area, a thirteenth area, a fourteenth area, a fifteenth area, and a sixteenth area. A seventh sub-diffraction light detection section is provided in the eleventh area. An eighth sub-diffraction light detection section is provided in the twelfth area. A ninth sub-diffraction light detection section is provided in the thirteenth area. A tenth sub-diffraction light detection section is provided in the fourteenth area. An eleventh tenth sub-diffraction light detection section is provided in the fifteenth area. A twelfth sub-diffraction light detection section is provided in the sixteenth area. The third diffraction light is detected by the seventh sub-diffraction light detection section and the eighth sub-diffraction light detection section. The fourth diffraction light is detected by the eighth sub-diffraction light detection section and the ninth sub-diffraction light detection section. The fifth diffraction light is detected by the tenth sub-diffraction light detection section and the eleventh sub-diffraction light detection section. The sixth diffraction light is detected by the eleventh sub-diffraction light detection section and the twelfth sub-diffraction light detection section. The control device obtains a focusing error signal for the optical disc based on a difference between a total light amount of the diffraction light detected by the seventh sub-diffraction light detection section, the ninth sub-diffraction light detection section and the eleventh sub-diffraction light detection section, and a total light amount of the sub-diffraction light detected by the eighth sub-diffraction light detection section, the tenth sub-diffraction light detection section, and the twelfth sub-diffraction light detection section.
Thus, the invention described herein makes possible the advantages of providing an optical disc apparatus for sufficiently decreasing the degree of asymmetry of a tracking error signal caused by the shift of the central axis of an objective lens with respect to the optical axis of the optical disc apparatus and suppressing off-track, so as to realize satisfactory and stable recording and reproduction.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.